The present invention relates generally to the fields of electrochemical cells and of separators for use in electrochemical cells. More particularly, this invention pertains to methods of preparing a cathode/separator assembly in which a microporous separator layer is coated on a temporary carrier substrate and a cathode active layer is then coated on the separator layer prior to removing the temporary carrier substrate from the separator layer. Also, this invention pertains to methods of preparing electrochemical cells utilizing cathode/separator assemblies prepared by the methods of this invention. The present invention also pertains to cathode/separator assemblies and electrochemical cells prepared by such methods.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
In an electrochemical cell or battery, discharge of the cell from its charged state occurs by allowing electrons to flow from the anode to the cathode through an external circuit resulting in the electrochemical reduction of the cathode active material at the cathode and the electrochemical oxidation of the anode active material at the anode. Under undesirable conditions, electrons may flow internally from the anode to the cathode, as would occur in a short circuit. To prevent this undesirable internal flow of electrons that occurs in a short circuit, an electrolyte element is interposed between the cathode and the anode. This electrolyte element must be electronically non-conductive to prevent the short circuiting, but must permit the transport of ions between the anode and the cathode during cell discharge, and in the case of a rechargeable cell, also during recharge. The electrolyte element should also be stable electrochemically and chemically towards both the anode and the cathode.
Typically, the electrolyte element contains a porous material, referred to as a separator since it separates or insulates the anode and the cathode from each other, and an aqueous or non-aqueous electrolyte in some or all of the pores of the separator. The aqueous or non-aqueous electrolyte typically comprises ionic electrolyte salts and water or electrolyte solvents, and optionally, other materials or additives such as, for example, ionically conductive polymers. A variety of materials have been used for the porous layer or separator of the electrolyte element in electrochemical cells. These porous separator materials include polyolefins such as polyethylenes and polypropylenes, glass fiber and paper filter papers, and ceramic materials. Usually these separator materials are supplied as porous free standing films which are interleaved with the anodes and the cathodes in the fabrication of electrochemical cells. Alternatively, the porous layer can be applied directly to one of the electrodes, for example, as described in U.S. Pat. No. 3,625,771 to Arrance et al., 5,194,341 to Bagley et al., and U.S. Pat. No. 5,882,721 and U.S. Pat. No. 5,948,464 to Delnick; and in Eur. Pat. Application Nos. 848,435 to Yamashita et al.; and 892,449 to Bogner.
Porous separator materials have been fabricated by a variety of processes including, for example, stretching combined with special heating and cooling of plastic films, extraction of a soluble plasticizer or filler from plastic films, and plasma oxidation. The methods for making free standing separators typically involve extrusion of melted polymeric materials either followed by a post-heating and stretching or drawing process or followed by a solvent extraction process to provide the porosity throughout the separator layer. U.S. Pat. No. 5,326,391 to Anderson et al. and references therein, describe the fabrication of free standing porous materials based on extraction of a soluble plasticizer from pigmented plastic films. U.S. Pat. No. 5,418,091 to Gozdz et al. and references therein, describe forming electrolyte layers by extracting a soluble plasticizer from a fluorinated polymer matrix either as a coated component of a multilayer battery structure or as a free standing separator film. U.S. Pat. No. 5,894,656 to Menon et al. describes forming an electrode directly on the surface of an electrolyte layer having a soluble plasticizer and then extracting the plasticizer to activate the battery. U.S. Pat. No. 5,194,341 to Bagley et al. describes an electrolyte element with a microporous silica layer and an organic electrolyte. The microporous silica layer was the product of the plasma oxidation of a siloxane polymer.
U.S. patent application Ser. No. 08/995,089 titled xe2x80x9cSeparators for Electrochemical Cells,xe2x80x9d filed Dec. 19, 1997 now U.S. Pat. No. 6,153,337, to Carlson et al. of the common assignee, describes separators for use in electrochemical cells which comprise a microporous pseudo-boehmite layer, and electrolyte elements and cells comprising such separators. The pseudo-boehmite separators and methods of preparing such separators are described for both free standing separators and as a separator layer coated directly onto an electrode.
When a separator layer is coated directly onto an electrode, such as onto the cathode, the porous separator coating may require a relatively smooth, uniform surface on the cathode and also may require a mechanically strong and flexible cathode layer. For example, for a microporous pseudo-boehmite layer having a xerogel structure, these cathode surface and layer properties may be required to prevent excessive stresses and subsequent cracking of the xerogel layer during drying of a pseudo-boehmite coating on the cathode surface and also during fabrication and use of electrochemical cells containing the pseudo-boehmite layer.
In addition to being porous and being chemically stable to other materials of the electrochemical cell, the separator should be flexible, thin, economical in cost, and have good mechanical strength. These properties are particularly important when a cell with thinner cathode, separator, and anode layers is spirally wound or is folded to increase the surface area of the electrodes and thereby increase the capacity and high rate capability of the cell. Typically, free standing separators for batteries have been 25 microns or greater in thickness. As batteries have continued to evolve to higher volumetric capacities and smaller lightweight structures, there is an increasing need for separators that are 15 microns or less in thickness with a substantial increase in the area of the separator contained in each particular size of battery. Reducing the thickness from 25 microns to 15 microns or less greatly increases the challenge of providing porosity and good mechanical strength while not sacrificing the protection against short circuits or not significantly increasing the total cost of the separator in each battery.
This protection against short circuits is particularly critical in the case of secondary or rechargeable batteries with lithium as the anode active material. During the charging process of the battery, dendrites may form on the surface of the lithium anode and may grow with continued charging. A key feature of the separator in the electrolyte element of lithium rechargeable batteries is that it have small pore structures, such as 1 micron or less in pore diameter, and sufficient mechanical strength to prevent the lithium dendrites from contacting the cathode and causing a short circuit with perhaps a large increase in the temperature of the battery leading to an unsafe explosive condition.
Further it would be advantageous to be able to prepare electrochemical cells having separators with ultrafine pores and a wide range of thicknesses coated in contact to another layer of the electrochemical cell by a process of coating without requiring any subsequent solvent extraction or other complex process which is costly, difficult to control, and not effective in providing interconnected ultrafine pores.
A method for preparing electrochemical cells having a separator, particularly a separator with a thickness less than 15 microns, which can avoid the foregoing problems often encountered with the use of porous polyolefinic and other conventional porous separator materials made using extrusion, extraction, or other conventional processes would be of great value to the battery industry.
The present invention pertains to methods of preparing a cathode/separator assembly for use in an electrochemical cell, wherein the cathode/separator assembly comprises a cathode active layer and a microporous separator layer, which methods comprise the steps of (a) coating a microporous separator layer on a temporary carrier substrate; (b) coating a cathode active layer in a desired pattern on a surface of the separator layer, which surface is on the side of the separator layer opposite from the temporary carrier substrate; and (c) removing the temporary carrier substrate from the separator layer to form the cathode/separator assembly. In one embodiment, the separator layer comprises one or more microporous xerogel layers. In one embodiment, the cathode/separator assembly further comprises one or more protective coating layers comprising a polymer, wherein the one or more protective coating layers are in contact with at least one of the one or more microporous xerogel layers of the separator layer. In one embodiment, one of the one or more microporous xerogel layers of the separator layer is coated directly on the temporary carrier substrate. In one embodiment, one of the one or more protective coating layers of the cathode/separator assembly is coated directly on the temporary carrier substrate, and one of the one or more microporous xerogel layers of the microporous separator layer is then coated on a surface of the one of the one or more protective coating layers, which surface is on the side of the one of the one or more protective coating layers opposite from the temporary carrier substrate, and further wherein the temporary carrier substrate is removed in step (c) from the surface of the one of the one or more protective coating layers, which surface is on the side of the one of the one or more protective coating layers opposite from the separator layer. In one embodiment, one of the one or more protective coating layers of the cathode/separator assembly is coated after step (a) directly on the surface of the separator layer, which surface is on the side of the separator layer opposite from the temporary carrier substrate layer, prior to coating the cathode active layer in step (b).
In a preferred embodiment of the methods of preparing a cathode/separator assembly of this invention, the separator layer comprises one or more microporous pseudo-boehmite layers. In a more preferred embodiment, the cathode/separator assembly further comprises one or more protective coating layers comprising a polymer, wherein the one or more protective coating layers are in contact with at least one of the one or more microporous pseudo-boehmite layers of the separator layer. In one embodiment, one of the one or more microporous pseudo-boehmite layers of the separator layer is coated directly on the temporary carrier substrate. In one embodiment, one of the one or more protective coating layers of the cathode/separator assembly is coated directly on the temporary carrier substrate, and one of the one or more microporous pseudo-boehmite layers of the microporous separator layer is then coated on a surface of the one of the one or more protective coating layers, which surface of the one of the one or more the protective coating layers is opposite from the temporary carrier substrate, and further wherein the temporary carrier substrate is removed in step (c) from the surface of the one of the one or more protective coating layers, which surface is opposite from the separator layer. In one embodiment, one of the one or more protective coating layers of the cathode/separator assembly is coated after step (a) directly on the surface of one of the one or more microporous pseudo-boehmite layers of the separator layer, prior to coating the cathode active layer in step (b).
In one embodiment of the methods of preparing a cathode/separator assembly of the present invention, the temporary carrier substrate is a flexible web substrate. Suitable web substrates include, but are not limited to, papers, polymeric films, and metals. In one embodiment, the flexible web substrate is surface treated with a release agent.
In one embodiment of the methods of preparing a cathode/separator assembly of this invention, the cathode active layer comprises an electroactive material selected from the group consisting of electroactive metal chalcogenides, electroactive conductive polymers, and electroactive sulfur-containing materials. In one embodiment, the cathode active layer comprises elemental sulfur. In one embodiment, the cathode active layer comprises an electroactive sulfur-containing organic polymer, wherein the polymer, in its oxidized state, comprises one or more polysulfide moieties selected from the group consisting of xe2x80x94Smxe2x80x94, xe2x80x94Smxe2x88x92, and Sm2xe2x88x92; where m is an integer equal to or greater than 3.
In one embodiment of the methods of preparing a cathode/separator assembly of the present invention, the desired pattern of the cathode active layer does not completely coat or cover the surface of the separator layer, which surface is on the side of the separator layer opposite from the temporary carrier substrate.
In one embodiment, the methods further comprise a step of coating an edge insulating layer in a desired pattern on the surface of the separator layer. In one embodiment, the step of coating the edge insulating layer occurs subsequent to the steps of coating the microporous separator and cathode active layers and prior to the step of removing the temporary carrier substrate from the separator layer. In one embodiment, the desired pattern of the edge insulating layer comprises substantially the remaining area of the surface of the separator layer that is not coated with the desired pattern of the cathode active layer. In one embodiment, a portion of the desired pattern of the edge insulating layer is in contact with a portion of the desired pattern of the cathode active layer. In one embodiment, the thickness of the edge insulating layer is substantially the same as the thickness of the cathode active layer. In one embodiment, the step of coating the edge insulating layer occurs subsequent to the step of coating the microporous separator layer and prior to the steps of coating the cathode active layer and removing the temporary carrier substrate from the separator layer. In one embodiment, the edge insulating layer comprises an insulating xerogel layer. In one embodiment, the edge insulating layer comprises an insulating non-porous, polymeric layer.
In one embodiment of the methods of preparing a cathode/separator assembly of this invention, the methods further comprise a step of depositing a cathode current collector layer in a desired pattern on the surface of the cathode active layer, which surface is opposite from the separator layer. In one embodiment, the step of depositing the cathode current collector layer occurs subsequent to the steps of coating the microporous separator and cathode active layers and prior to the step of removing the temporary carrier substrate from the separator layer. In one embodiment, the methods further comprise a step of coating an electrode insulating layer in a desired pattern on the surface of the cathode current collector layer, which surface is opposite from the separator layer.
In one embodiment of the methods of preparing a cathode/separator assembly of the present invention, the methods further comprise a step of depositing a cathode current collector layer in a desired pattern on the outer surface of the cathode active layer and, optionally, on the outer surface of the edge insulating layer. In one embodiment, the step of depositing the cathode current collector layer occurs subsequent to the steps of coating the microporous separator, cathode active, and edge insulating layers, and prior to the step of removing the temporary carrier substrate from the separator layer. In one embodiment, the step of depositing the cathode current collector layer occurs subsequent to the steps of coating the microporous separator and cathode active layers, prior to the step of coating the edge insulating layer, and prior to the step of removing the temporary carrier substrate from the separator layer. In one embodiment, the methods further comprise a step of coating an electrode insulating layer in a desired pattern on the outer surface of the cathode current collector layer and, optionally, on the outer surface of the edge insulating layer.
Another aspect of the present invention pertains to methods of preparing an electrochemical cell, which methods comprise the steps of: (a) providing a cathode/separator assembly prepared by a method comprising the steps of (i) coating a microporous separator layer on a temporary carrier substrate, (ii) coating a cathode active layer in a desired pattern on a surface of the separator layer, which surface is on the side of the separator layer opposite from the temporary carrier substrate, and (iii) removing the temporary carrier substrate from the separator layer to form the cathode/separator assembly; (b) providing an anode; (c) providing a cathode current collector layer; (d) providing an electrode insulating layer interposed between the anode and the cathode current collector layer; and (e) providing an electrolyte, wherein the electrolyte is contained in pores of the separator layer; and wherein the separator layer of the cathode/separator assembly and the anode are positioned in a face-to-face relationship and the cathode active layer and the cathode current collector layer are positioned in a face-to-face relationship. In one embodiment, the separator layer comprises one or more microporous xerogel layers. In one embodiment, the cathode/separator assembly further comprises one or more protective coating layers comprising a polymer, wherein the one or more protective coating layers are in contact with at least one of the one or more microporous xerogel layers of the separator layer.
In a preferred embodiment of the methods of preparing an electrochemical cell of this invention, the separator layer comprises one or more microporous pseudo-boehmite layers. In a more preferred embodiment, the electrochemical cell further comprises one or more protective coating layers comprising a polymer, wherein the one or more protective coating layers are in contact with at least one of the one or more microporous pseudo-boehmite layers of the separator layer.
In one embodiment of the methods of preparing an electrochemical cell of this invention, the anode comprises an anode active material selected from the group consisting of lithium metal, lithium-aluminum alloys, lithium-tin alloys, lithium-intercalated carbons, and lithium-intercalated graphites. Suitable electrolytes include liquid electrolytes, gel polymer electrolytes, and solid polymer electrolytes. In one embodiment, the electrolyte comprises a liquid electrolyte.
In one embodiment of the methods of preparing an electrochemical cell of the present invention, the electrode insulating layer comprises a polymeric plastic film. In one embodiment, the electrode insulating layer comprises a polymeric coating.
In one embodiment of the methods of preparing an electrochemical cell of this invention, the cell is a secondary cell. In one embodiment of the methods of preparing an electrochemical cell of this invention, the cell is a primary cell.
Another aspect of the present invention pertains to methods of preparing an electrochemical cell, which methods comprise the steps of: (a) providing a cathode/separator assembly prepared by a method comprising the steps of (i) coating a microporous separator layer on a temporary carrier substrate, (ii) coating a cathode active layer in a desired pattern on a surface of the separator layer, which surface is on the side of the separator layer opposite from the temporary carrier substrate, (iii) coating an edge insulating layer in a desired pattern on the surface of the separator layer, and (iv) removing the temporary carrier substrate from the separator layer to form the cathode/separator assembly; (b) providing an anode; (c) providing a cathode current collector layer; (d) providing an electrode insulating layer interposed between the anode and the cathode current collector layer; and (e) providing an electrolyte, wherein the electrolyte is contained in pores of the separator layer; and wherein the separator layer of the cathode/separator assembly and the anode are positioned in a face-to-face relationship, and the cathode active layer and the cathode current collector layer are positioned in a face-to-face relationship.
Another aspect of this invention pertains to methods of preparing an electrochemical cell comprising a casing and a multilayer cell stack, which methods comprise the steps of: (a) providing a laminar combination of (i) an anode assembly comprising an anode comprising an anode active layer, wherein the anode active layer comprises an anode active material comprising lithium; a first cathode current collector layer; and an electrode insulating layer interposed between the anode and the first cathode current collector layer; and (ii) a cathode/separator assembly comprising a cathode active layer in a first desired coating pattern on a surface of a microporous separator layer and with an edge insulating layer in a second desired coating pattern on the surface of the separator layer; wherein the first cathode current collector layer and the cathode active layer are positioned in a face-to-face relationship; (b) winding the laminar combination to form an anode-electrode insulating layer-first cathode current collector layer-cathode/separator assembly multilayer cell stack, wherein the first cathode current collector layer is in contact with the cathode active layer; (c) providing an electrolyte, wherein the electrolyte is contained in pores of the separator layer; (d) providing a casing to enclose the cell stack; and (e) sealing the casing. In one embodiment, the anode further comprises an anode current collector layer interposed between the anode active layer and the electrode insulating layer. In one embodiment, a second cathode current collector layer is deposited in a third desired pattern on the outer surface of the cathode active layer and on the outer surface of the edge insulating layer.
In one embodiment of the methods of preparing an electrochemical cell of this invention, the cathode/separator assembly of step (a) further comprises a temporary carrier substrate on a surface of the separator layer, which surface is on the side of the separator layer opposite from the cathode active layer and the edge insulating layer, and the methods further comprise a step of removing the temporary carrier substrate from the separator layer prior to completion of step (b). In one embodiment, a second cathode current collector layer is deposited in a third desired coating pattern on the outer surface of the cathode active layer and on the outer surface of the edge insulating layer.
In one embodiment of the methods of preparing an electrochemical cell of the present invention of the methods of preparing an electrochemical cell of the present invention, the anode of the anode assembly and the separator layer of the cathode/separator assembly are positioned in a face-to-face relationship in step (a), and a first cathode current collector layer-electrode insulating layer-anode-cathode/separator assembly multilayer cell stack is formed in step (b). In one embodiment, a second cathode current collector layer is deposited in a third desired coating pattern on the outer surface of the cathode active layer and on the outer surface of the edge insulating layer.
In one embodiment of the methods of preparing an electrochemical cell of this invention, the cathode/separator assembly of step (a) further comprises a temporary carrier substrate on a surface of the separator layer, which surface is on the side of the separator layer opposite from the cathode active layer and the edge insulating layer, and the methods further comprise a step of removing the temporary carrier substrate from the separator layer prior to completion of step (b). In one embodiment, a second cathode current collector layer is deposited in a third desired coating pattern on the outer surface of the cathode active layer and on the outer surface of the edge insulating layer.
Another aspect of this invention pertains to methods of preparing an electrochemical cell comprising a casing and a multilayer cell stack, which methods comprise the steps of: (a) providing a laminar combination of (i) an anode assembly comprising an anode comprising lithium metal; and (ii) a cathode/separator assembly comprising a cathode active layer in a first desired coating pattern on a surface of a microporous separator layer and further comprising an edge insulating layer in a second desired coating pattern on the surface of the separator layer; a cathode current collector layer in a third desired coating pattern on the outer surface of the cathode active layer and on the outer surface of the edge insulating layer; an electrode insulating layer in a fourth desired coating pattern on the outer surface of the cathode current collector layer and on the outer surface of the edge insulating layer; wherein the anode and the electrode insulating layer are positioned in a face-to-face relationship; (b) winding the laminar combination to form an anode-electrode insulating layer-cathode current collector layer-cathode/separator assembly multilayer cell stack, wherein the anode is in contact with the separator layer; (c) providing an organic electrolyte, wherein the organic electrolyte is contained in pores of the separator layer; (d) providing a casing to enclose the cell stack; and (e) sealing the casing. In one embodiment, the cathode/separator assembly of step (a) further comprises a temporary carrier substrate on a surface of the separator layer, which surface is on the side of the separator layer opposite from the cathode active layer and the edge insulating layer, and the methods further comprise the step of removing the temporary carrier substrate from the separator layer prior to completion of step (b). In one embodiment, the anode and the separator layer of the cathode/separator assembly are positioned in a face-to-face relationship in step (a), and an anode-cathode/separator assembly-cathode current collector layer-electrode insulating layer multilayer cell stack is formed in step (b).
Another aspect of this invention pertains to cathode/separator assemblies prepared according to the methods of this invention, as described herein. Another aspect of the present invention pertains to electrochemical cells prepared according to the methods of the present invention, as described herein, and comprising a cathode/separator assembly, as described herein.
As will be appreciated by one of skill in the art, features of one aspect or embodiment of the invention are also applicable to other aspects or embodiments of the invention.